1. Field of the Invention
This invention relates to the field of wireless communication systems. More specifically, the invention relates to an RF transceiver circuit for a dual mode mobile unit.
2. Description of the Related Art
The use of wireless communication systems is growing with users now numbering well into the millions. In one type of wireless communication system, the cellular network, a mobile unit communicates with an associated base station at a predefined band of radio frequencies. The base station in turn communicates with a land based telephone system, effectively linking the mobile unit to the land based telephone system.
As a mobile unit travels through a cellular network, the mobile unit moves through service areas known as cells. Each cell is a specific geographic region containing a base station. When moving from one cell to another, the base station servicing the mobile unit changes from the base station of one cell to the base station of another cell.
The number of companies competing to offer wireless communications is growing. In an effort to offer differing products, and due to FCC regulations limiting the number of licenses at a specific band of frequencies, the cellular systems offered operate on a variety of different bands of frequencies. When leaving one cellular system and moving into another, the frequency at which the base stations operate may change. Because most original mobile units were designed to operate only within a set band of frequencies, the mobile units could not function with both cellular systems, each operating at different sets of frequency bands. For example, a mobile unit designed for use with a system using the Global System for Mobile communications (GSM) operating at a band of frequencies around 900 MHz would not function in a second cellular system using the Digital Cellular System (DCS) operating at a band of frequencies around 1800 MHz. In particular, each band of frequencies for both the GSM system and the DCS system is divided into channels, and any specific transmission is accomplished within a channel. Each channel has a width of 200 kilohertz (kHz).
Dual-mode mobile units were created to allow one mobile unit to be more flexible and have the ability to operate with multiple cellular systems. It should be kept in mind that one company may offer service in two separate bands, offering those customers with dual-mode mobile units more channels to choose from. These original dual-mode mobile units sometimes contained a separate oscillator and associated circuitry to generate each separate frequency. Although this allowed the mobile unit to cross-over to a second cellular system, the addition of the second oscillator and associated circuitry increased the size, cost, and complexity of the mobile unit. Because mobile units are designed to be portable, it is also always desirable to reduce the size of the mobile unit.
What is needed is a mobile unit capable of operating at two separate frequencies without the increasing the size and cost of the mobile unit from that of the single mode mobile unit.
In one embodiment of the present invention a wireless telephone is provided which is capable of operation at two different bands of radio frequencies. The wireless telephone generates the two frequencies bands using only a single phase lock loop. Because only one phase lock loop is used to generate the two frequencies, the wireless telephone may be constructed with only a single local oscillator. Reducing the number of local oscillators decreases the physical size of the wireless telephone and reduces the cost of manufacture. Further, a lower frequency oscillator can be used. This also reduces cost because an accurate, high quality, low frequency oscillator is less expensive than a higher frequency oscillator with similar accuracy and quality.
One embodiment of the invention is a dual-mode telephone having a single phase lock loop that can select between two output frequency bands. A switch in the phase lock loop selectively swaps an UP signal and a DOWN signal to achieve either a high side lock or a low side lock. When the phase lock loop is high side locked, one of the frequencies in the higher band of output frequencies may be generated. When the phase lock loop is low side locked, one of the frequencies in the lower band of output frequencies may be generated.
Another embodiment of the invention is a dual-mode wireless telephone having a phase lock loop with two selectable bands of output frequencies. The dual-mode wireless telephone includes a mixer which receives a local oscillator signal and an output signal. The mixer then generates a delta signal. A phase comparator compares the delta signal with an intermediate signal and generates an UP signal and a DOWN signal based on the results of the comparison. A switch having a first output and a second output receives the UP signal and the DOWN signal. In a first position, the switch outputs the UP signal on the first output and the DOWN signal on the second output. In a second position, the switch outputs the UP signal on the second output and the DOWN signal on the first output. A signal generator then converts the UP signal and the DOWN signal into a control signal, which is used by a voltage controlled oscillator to adjust the frequency of the output signal.
Another embodiment of the invention is a method of having either a high side lock or a low side lock from a phase lock loop. The method includes the steps of combining a local oscillator signal with an output signal to create a combined signal, and then comparing the combined signal with an intermediate signal. Based on the comparison of the first combined signal and the intermediate signal, an UP signal is generated on a first signal line and a DOWN signal is generated on a second signal line. The method further includes the steps of selecting either a high side lock or a low side lock of the phase lock loop, and switching the UP signal to the second signal line and the DOWN signal to the first signal line if the low side lock is selected. A control signal is then generated based on the values of the signals on the first signal line and the second signal line and the frequency of the output signal is adjusted based on the control signal.
Another embodiment of the invention is a phase lock loop capable of selectively generating an output signal operating within one of two bands of frequencies. The phase lock loop includes a phase detector which generates a first signal and a second signal and a signal generator which generates a control signal based on the signals from the phase detector. The phase lock loop also includes an output frequency selector having a first position and a second position. In the first position, the output frequency selector transfers the first signal to a first input of the signal generator and transfers the second signal to a second input of the signal generator. In a second position, the output frequency selector transfer the first signal to the second input of the signal generator and transfers the second signal to the first input of the signal generator. An oscillator receives the control signal and adjusts the frequency of the output signal based on the control signal.
Another embodiment of the invention is a dual-mode wireless telephone having a single phase lock loop with a selectable output frequency band. The dual-mode wireless telephone includes means for selecting the desired output frequency band and means for locking the phase lock loop on either a high side or a low side. The choice of the high side lock or the low side lock is determined by the selecting means. The dual-mode telephone also includes means for generating a first output signal within a first band of frequencies when the phase lock loop is high side locked, and means for generating a second output signal within a second band of frequencies when the phase lock loop is low side locked.